Building quality & signal integrity into PoP-based PCB design & assembly
The characteristics of a dip unit are key considerations during the early phases of design because it is crucial for perfect PoP assemblies. Consequently, it is of utmost importance to the OEM.
The dip unit is used to apply the correct amount of tacky flux or paste used in the PoP placement operation. For example, MYDATA’s dip unit is fully automated with no manual adjustments needed. During the pre-production phase, the amount of flux or solder needed is verified, which is relatively easy for automated dip units, but slightly challenging for manual ones.
Also, the amount of tacky flux or solder paste must be consistent. The dip unit must deposit the same amount of paste or tacky flux to uniformly adhere to every BGA ball. Bridges or shorts are created when too much flux is used, whereas opens and cold solders appear when too little is used. As for the dipping depth, the normal range is 50% to 66% of the height of the ball for a time interval or dwell time of 0.1 to 1.0 seconds. For paste, it is about 50% of the height of the ball; whereas for flux, it is 66%.
An ideal dip unit is flexible enough to allow automatic changes in flux or paste thickness and provides support for different flip-chip bump sizes. These product traits are especially valuable to stay ahead of an ever-evolving PoP technology.
Moving To Next PoP Generations
The industry will face challenges as it moves into newer generations, meaning 0.4mm pitch BGAs and below, with PoP stacks of more than two BGAs. The challenges of PoP assembly in a pick and place (P&P) machine are basically found in two distinct and independent process steps: solder paste application and placement.
It is important to emphasize that all major fluid suppliers have developed paste and flux optimized for the dipping process. This means that a thin layer can be applied on a flat surface using a squeegee or something similar. For example, experience shows that the MYDATA Dip Unit works well with commercially available fluids, doesn’t require a customized fluid, and works equally well with both tacky flux and dip paste. MYDATA has experience with both types of fluids. The conclusion is that the basic mechanical design with a transfer plate with cavities and a squeegee is a good solution for both applications.
As for the second independent process step, for a P&P machine to be able to place a PoP device, it must fulfill some basic requirements, such as accuracy, programmable z-level, and post-dipping vision recognition.
High-end P&P machine suppliers shouldn't have any major issues with these basic capabilities. However, they may have chosen different approaches in handling the varying z-level. For a more robust PoP process that can handle larger variations in part height, a fast z-level impact sensor on the nozzle is preferred, compared to force feedback using servo motor current sensing.
If the placement process is well within the capabilities of today’s high-end equipment, the paste application is where different solutions can have a large impact on the results. The process window is quite small for a successful paste application using the dipping method.
A dipping depth of 50-66% of the ball size is a typical requirement for BGA type components. It's obvious that this is hard to reach with a process that is relying on mechanical properties of a complex fluid such as solder paste.
In addition, even small variations -- for example, part co-planarity, ball size, and operator skills -- can jeopardize quality. These difficulties will be even greater when going to next-generation PoP with smaller pitch and ball size. To stay on the short-list for PoP assembly, equipment suppliers must provide a solution that is operator independent and as forgiving as possible regarding material variations.
Second/Third Generation PoP Placement
The popular belief is that a pick and place machine requires certain advanced features and attributes to effectively handle second and third generation PoP placement. However, that’s a faulty assumption.
First of all, when designing an efficient PoP assembly process, it’s important to look not just at the pick and place machine, but the entire line. Savings in one step may incur additional costs in other steps, so a holistic view is required. The next generation of PoP will certainly demand higher levels of accuracy and reliability from pick and place machines, but the industry is facing an even bigger challenge when it comes to the paste application.
The future development of PoP devices heavily depends on the equipment vendor’s ability to develop solutions for highly reliable paste application with outstanding volume repeatability. When placing fine-pitch BGAs with thousands of balls, even a very small increase in defect rate will quickly become a major issue on final assembly level.
The most critical parameter affecting overall yield is the volume control or how much paste is actually transferred to each ball. Not only the absolute volume, but also the ball-to-ball variation is important to understand. Too much variation will lead to an open circuit, or even worse, an inferior joint or latent defect that fails later in the field.
The dipping process has inherent difficulties dealing with volume control. The paste transfer is relying on hard-to-control parameters such as tackiness and paste surface flatness. Also, it is important to note that post-dipping inspection of paste volume is probably not even possible with the type of 2D vision systems that are generally built into today's P&P machines. 3D inspection from below could be a way to take the dipping process one step further, but it's likely that the next-generation PoP will force the industry to look for alternative paste application methods.
One such method could be to deposit paste on the bottom component, prior to placing the top component. The main candidates for such a process are dispensing and jetting. While volume control and accuracy are probably more or less comparable, jetting would have the advantage of higher speed, making it the preferred choice in high-volume manufacturing.
Besides the improved volume control, both these methods also allow for 3D inspection of the paste deposit prior to placement of the top component. In-depth studies on the process yield are needed to determine if such a mid-process inspection will actually be necessary to reach the industry's quality requirements.
Zulki Khan is the Founder and President of NexLogic Technologies, Inc., San Jose, CA, an ISO 9001:2008 Certified Company, ISO 13485 certified for medical electronics, and a RoHS compliant EMS provider. Prior to NexLogic, he was General Manager for Imagineering, Inc., Schaumburg, IL. He has also worked on high-speed PCB designs with signal integrity analysis. He holds a B.S.E.E from N.E.D University and an M.B.A from University of Iowa and is a frequent author of contributed articles to EMS industry publications.
Mattias Jonsson is Product Manager in the SMT Business Unit at Micronic Mydata AB, Stockholm, Sweden. Micronic Mydata develops and manufactures equipment for SMT assembly, as well as laser-based pattern generators for manufacturing of displays, substrates and semiconductors. He has also held positions in Electrical Design and Project Management. He holds an M.Sc. degree in Electrical Engineering from the Royal Institute of Technology (KTH) in Stockholm.
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